fferent hills near Krokee,
or in Mount Taygetus in Lacedaemon, where the old quarry has recently
been opened. It has a base of dark green with angular crystals of
felspar of a lighter green imbedded in it. It is a truly eruptive
rock, occurring in intrusive bosses, or in beds interstratified with
gneiss and mica-schist, and owes its various shades of green to the
presence of copper. Owing to its extraordinary hardness, this stone
was seldom used for architectural purposes; and the lapidary will
charge three times as much for working a fragment of this material
into a letter-weight as for making it of any other stone. A pair of
fluted Roman Ionic columns, supporting the pediment of the altar of
the chapel of St. John the Baptist, in the Baptistery of St. John
Lateran, are the only examples of ophite pillars in Rome. Next to
these the largest masses are a circular tablet, forming part of the
splendid sheathing of one of the ambones in the Church of San Lorenzo;
and two elliptical tablets, still larger, engrafted upon the pilasters
in front of the high altar of St. Paul's.

The principal use to which this stone was devoted in Rome was the
construction of mosaic pavements. The emperor Alexander Severus
introduced into his palaces and public buildings a kind of flooring
composed of small squares of green serpentine and red porphyry,
wrought into elegant patterns, which became very fashionable, and was
called after himself _Opus Alexandrinum_. The infamous Heliogabalus
had previously paved some of the courts of the Palatine with such
intarsio work, but his cousin Alexander Severus, following his
example, adorned with it all the terraces and walks around, and the
pavements within, the isolated villas called Diaetae, dedicated to his
mother Mammaea, which he added to the Palatine buildings. We have
examples of this beautiful kind of tesselated pavement in some of the
chambers of the Baths of Caracalla; and it is highly probable that the
_Opus Alexandrinum_ in the transept and middle nave of the Church of
Santa Maria in Trastevere is in part at least contemporaneous with
Alexander Severus, who conceded the ground on which the original
oratory stood to Pope Calixtus I. in 222, for the special use of the
Christians. If this be so, we have in this first place of Christian
worship established in Rome the first instance of the application of
_Opus Alexandrinum_ to the decoration of a church. In the middle ages
the fashion was beauti